The present invention relates to capstans for driving a web-like material and in particular to a selectively moveable capstan assembly for driving a magnetic media in a data record/reproduce apparatus.
An inherent requirement for any transport apparatus whose function is to drive a magnetic media and/or similar flexible web-like materials includes some form of capstan and capstan motor for accurately transporting and metering the media at a controlled velocity, in either direction. In general, at present there are two types of capstan assemblies typically in common use in the field of data record/reproduce apparatus.
A first type of capstan assembly is known as a pinch roller/capstan combination, in which a media or web is threaded between a cylindrical capstan and a translatable pinch roller which usually includes a peripheral cylindrical surface of a resilient material. During controlled media motion, which typically is -1 to +3 times normal play speed in a magnetic record/reproduce apparatus, the media is urged into contact with the capstan by translating the pinch roller into contact with the capstan with a solenoid, lever, or other means. Such a capstan assembly requires little or no wrap of the media about the capstan since the media is firmly pressed against the capstan by the pinch roller to thereby prevent the possibility of slippage between the media and driving capstan. This assembly also permits decoupling the media or web from the capstan in situations where the media is transported at relatively high speeds such as, for example, during a shuttle mode of operation of a record/reproduce apparatus, whereby the capstan does not have to be capable of operating at such high speeds. However, since the capstan is not rotating during such high speed operating modes, it is necessary to derive media speed information from some form of tachometer-equipped timer idler disposed in contact with the media elsewhere along the media path. Such information is needed in order to monitor the amount of media remaining on the media reels, the end of the media, etc.
The above pinch roller/capstan assembly has various disadvantages which include, for example, the need for the additional timer idler to which the tachometer is coupled. Any additional idlers or guides in contact with the media have the potential of causing additional wear, and add to the complexity of the transport design and fabrication. Another disadvantage is the loss of media motion control during high speed media winding modes since the high performance capstan and its associated capstan servo circuit is not available. A still further disadvantage is that the pinching of the media between the capstan and the pinch roller can cause media surface damage, especially if the magnetic surface of, for example, a magnetic media faces the capstan. Thus media surface damage may occur in a pinch roller/capstan assembly due to skidding during media acceleration as well as from surface roughness and contaminants.
A second type of capstan assembly is known as a direct coupled capstan in which a media or web is threaded around a capstan hub and maintains contact with the capstan in all modes of media transport operation. In this assembly, the capstan drives the media during all operational speeds, including high speed media winding modes, in both directions. Typically, such capstans are equipped with means to assure good media contact, which may include some form of a grooved surface to help expel any air entrapped between the media and the capstan, orifices coupled to a vacuum source to encourage media coupling, a resilient material on the peripheral tape bearing surface of the capstan to increase friction with the media, etc. Such capstans typically are of as large a diameter as practical in order to reduce the rotational speed of the capstan when operating at high media speeds. In addition, the media wrap angle on the capstan must be as large as possible to increase friction and minimize slippage, typically near 180 degrees.
The direct coupled capstan assembly has several disadvantages, the first of which is the need for a capstan drive motor capable of high rotational speeds, yet capable of accurate control in the -1 to +3 times normal play speeds. Such requirements add to the complexity and costs of the capstan/motor assembly. Another disadvantage is the increased motor wear resulting from the high speed operation. Still other disadvantages are the media path layout restrictions which result from both the large capstan diameter and the large wrap angle requirements, and further from the need to provide a media path layout wherein the capstan is located on the side of the media opposite to the magnetic surface of the media to prevent media damage. This is a particular concern and problem in cassette-loaded record/reproduce apparatus. Still another problem and associated disadvantage may occur in some types of video record/reproduce apparatus wherein the media layout is such that in order to achieve a large wrap angle on the capstan, two guideposts must be positioned adjacent to the capstan with undesirably large media wrap angles on each guidepost. Such a guidepost arrangement is practical only with relatively sophisticated airlubricated systems of added complexity, and would not be practical or affordable in less sophisticated, less costly transports.
It would be highly desirable to provide a capstan assembly which combines the advantageous features of each of the types of capstan assemblies of previous discussion, while circumventing as many of the disadvantages as possible.